Theoretical calculations have shown that the high-frequency scattering
from a thin spherical shell is dominated by a single peak that occurs near the
opening of the first symmetric Lamb wave. This peak is associated with a region
of strong negative group velocity surrounding the opening of the first
symmetric Lamb wave. Experimental verification of the existence of the
thickness quasiresonance on a 2% stainless steel shell is given. In addition an
analysis of the thickness quasiresonance in the complex-ka plane is presented
for shells of different material parameters. It is shown that the thickness
quasiresonance bifurcates as a function of Poisson's ratio in such a manner
that the region of negative group velocity remains close to the opening of the
first symmetric Lamb wave. [Work supported by ONR.]